📝 document wrapper sink types

docs/api/buffered-sink-type.md

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+---
+name: buffered-sink-type
+group: api
+category: sink
+update-time: 20260613
+description: Public buffered sink type used for threshold-based synchronous batching over another sink.
+key-word:
+    - sink
+    - buffer
+    - type
+    - public
+---
+
+## Buffered-sink-type
+
+`BufferedSink[S]` is the public buffering sink type used for threshold-based synchronous batching over another sink. It is the concrete sink type returned by `buffered_sink(...)` and preserves the wrapped sink type in its type parameter.
+
+### Interface
+
+```moonbit
+pub struct BufferedSink[S] {
+  sink : S
+  buffer : Ref[Array[Record]]
+  flush_limit : Int
+}
+```
+
+#### output
+
+- `BufferedSink[S]` - Public synchronous sink type that buffers records before forwarding them to the wrapped sink.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This is a public root struct, not a type alias.
+- The current fields are `sink : S`, `buffer : Ref[Array[Record]]`, and `flush_limit : Int`.
+- `buffered_sink(...)` constructs this type directly from a wrapped sink and flush threshold.
+- The sink preserves the wrapped sink type `S`, which is useful when typed composition still matters after buffering is introduced.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need A Typed Buffered Sink Value
+
+When code should keep the concrete buffering sink type visible:
+```moonbit
+let sink : BufferedSink[ConsoleSink] = buffered_sink(console_sink(), flush_limit=2)
+```
+
+In this example, the sink value stays explicit and preserves the wrapped console sink type.
+
+#### When Need A Typed Logger With Synchronous Batching
+
+When logging should preserve the buffering sink type in the logger:
+```moonbit
+let logger : Logger[BufferedSink[ConsoleSink]] = Logger::new(
+  buffered_sink(console_sink(), flush_limit=2),
+  target="buffered",
+)
+```
+
+In this example, the concrete buffered sink remains part of the logger type.
+
+### Error Case
+
+e.g.:
+- `BufferedSink[S]` itself does not have a runtime failure mode.
+
+- Runtime behavior still depends on the wrapped sink `S`, and buffered records can remain pending if flushing never happens.
+
+### Notes
+
+1. Use `buffered_sink(...)` when you need a value of this type.
+
+2. Use `QueuedSink[S]` when overflow-aware queueing behavior is needed instead of simple buffering.